Solutions to Selected Text Problems
chapter 2: atoms, molecules, and ions
chapter 2
atoms, molecules, and ions
2.8First, convert 1 cm to picometers.
2.9Note that you are given information to set up the unit factor relating meters and miles.
2.14For iron, the atomic number Z is 26. Therefore the mass number A is:
A 26 28 54
2.15Strategy:The 239 in Pu-239 is the mass number. The mass number (A) is the total number of neutrons and protons present in the nucleus of an atom of an element. You can look up the atomic number (number of protons) on the periodic table.
Solution:
mass number number of protons number of neutrons
number of neutrons mass number number of protons 239 94 145
2.16Isotope
No. Protons221212223578
No. Neutrons1212132644117
2.17Isotope
No. Protons7162938567480
No. Electrons7162938567480
No. Neutrons817344674112122
2.18(a)(b) (c) (d)
2.19The accepted way to denote the atomic number and mass number of an element X is
where A mass number and Z atomic number.
(a)(b) (c) (d)
2.20(a) 10(b) 26 (c) 81 (d)196
2.21(a) 19(b) 34 (c) 75 (d)192
2.22198Au: 119 neutrons, 47Ca: 27 neutrons, 60Co: 33 neutrons, 18F: 9 neutrons, 125I: 72 neutrons, 131I: 78 neutrons, 42K: 23 neutrons, 43K: 24 neutrons, 24Na: 13 neutrons, 32P: 17 neutrons, 85Sr: 47 neutrons, 99Tc: 56 neutrons.
2.28Helium and Selenium are nonmetals whose name ends with ium. (Tellurium is a metalloid whose name ends in ium.)
2.29(a)Metallic character increases as you progress down a group of the periodic table. For example, moving down Group 4A, the nonmetal carbon is at the top and the metal lead is at the bottom of the group.
(b)Metallic character decreases from the left side of the table (where the metals are located) to the right side of the table (where the nonmetals are located).
2.30The following data were measured at 20C.
(a)Li (0.53 g/cm3)K (0.86 g/cm3)H2O (0.98 g/cm3)
(b)Au (19.3 g/cm3)Pt (21.4 g/cm3)Hg (13.6 g/cm3)
(c)Os (22.6 g/cm3)
(d)Te (6.24 g/cm3)
2.31Na and K are both Group 1A elements; they should have similar chemical properties. N and P are both Group 5A elements; they should have similar chemical properties. F and Cl are Group 7A elements; they should have similar chemical properties.
2.32I and Br (both in Group 7A), O and S (both in Group 6A), Ca and Ba (both in Group 2A)
2.33
Atomic number 26, iron, Fe, (present in hemoglobin for transporting oxygen)
Atomic number 53, iodine, I, (present in the thyroid gland)
Atomic number 11, sodium, Na, (present in intra- and extra-cellular fluids)
Atomic number 15, phosphorus, P, (present in bones and teeth)
Atomic number 16, sulfur, S, (present in proteins)
Atomic number 12, magnesium, Mg, (present in chlorophyll molecules)
2.38(34.968 amu)(0.7553) (36.956 amu)(0.2447) 35.45 amu
2.39(203.973020 amu)(0..014) (205.974440 amu)(0.241) +(206.975872 amu)(0.221) (207.976627
amu)(0.524) 207.2 amu
2.40The fractional abundances of the two isotopes of Tl must add to 1. Therefore, we can write
(202.972320 amu)(x) (204.974401 amu)(1x) 204.4 amu
Solving for x gives 0.2869. Therefore, the natural abundances of 203Tl and 205Tl are 28.69% and 71.31%, respectively.
2.41Strategy: Each isotope contributes to the average atomic mass based on its relative abundance. Multiplying the mass of an isotope by its fractional abundance (not percent) will give the contribution to the average atomic mass of that particular isotope.
It would seem that there are two unknowns in this problem, the fractional abundance of 6Li and the fractional abundance of 7Li. However, these two quantities are not independent of each other; they are related by the fact that they must sum to 1. Start by letting x be the fractional abundance of 6Li. Since the sum of the two fractional abundances must be 1, we can write
(6.0151 amu)(x) (7.0160 amu)(1x) 6.941 amu
Solution: Solving for x gives 0.075, which corresponds to the fractional abundance of 6Li. The fractional abundance of 7Li is (1 x) = 0.925. Therefore, the natural abundances of 6Li and 7Li are 7.5% and 92.5%, respectively.
2.42The conversion factor required is
2.43The conversion factor required is
2.53(a)This is a polyatomic molecule that is an elemental form of the substance. It is not a compound.
(b)This is a polyatomic molecule that is a compound.
(c)This is a diatomic molecule that is a compound.
2.54(a)This is a diatomic molecule that is a compound.
(b)This is a polyatomic molecule that is a compound.
(c)This is a polyatomic molecule that is the elemental form of the substance. It is not a compound.
2.55Elements:N2, S8, H2
Compounds:NH3, NO, CO, CO2, SO2
2.56There are more than two correct answers for each part of the problem.
(a)H2 and F2(b)HCl and CO(c)S8 and P4
(d)H2O and C12H22O11 (sucrose)
2.57Strategy:An empirical formula tells us which elements are present and the simplest whole-number ratio of their atoms. Can you divide the subscripts in the formula by some factor to end up with smaller whole-number subscripts?
Solution:
(a)Dividing both subscripts by 2, the simplest whole number ratio of the atoms in C2N2 is CN.
(b)Dividing all subscripts by 2, the simplest whole number ratio of the atoms in C6H6 is CH.
(c)The molecular formula as written, C9H20, contains the simplest whole number ratio of the atoms present. In this case, the molecular formula and the empirical formula are the same.
(d)Dividing all subscripts by 2, the simplest whole number ratio of the atoms in P4O10 is P2O5.
(e)Dividing all subscripts by 2, the simplest whole number ratio of the atoms in B2H6 is BH3.
2.58(a)AlBr3(b)NaSO2(c)N2O5 (d)K2Cr2O7
2.59C3H7NO2
2.60C2H6O (The formula for ethanol can also be written as C2H5OH or CH3CH2OH.)
2.61(a) nitrogen trichloride (b) iodine heptafluoride (c)tetraphosphorus hexoxide
(d) disulfur dichloride
2.62(a) PBr3 (b) N2F4 (c)XeO4(d) SeO3
2.63All of these are molecular compounds. We use prefixes to express the number of each atom in the molecule.
The molecular formulas and names are: (a) NF3: nitrogen trifluoride (b) PBr5: phosphorus pentabromide (c) SCl2: sulfur dichloride
2.64(a) OF2: oxygen difluoride (b) Al2Br6: dialuminum hexabromide
(c) N2F4: dinitrogen tetrafluoride (also “perfluorohydrazine”)
2.69The atomic number (Z) is the number of protons in the nucleus of each atom of an element. You can find this on a periodic table. The number of electrons in an ion is equal to the number of protons minus the charge on the ion.
number of electrons (ion) number of protons charge on the ion
IonNaCa2Al3Fe2IFS2O2N3
No. protons112013265391687
No. electrons101810245410181010
2.70
IonKMg2Fe3BrMn2C4Cu2
No. protons1912263525629
No. electrons18102336231027
2.71(a)Sodium ion has a 1 charge and oxide has a 2 charge. The correct formula is Na2O.
(b)The iron ion has a 2 charge and sulfide has a 2 charge. The correct formula is FeS.
(c)The correct formula is Co2(SO4)3.
(d)Barium ion has a 2 charge and fluoride has a 1 charge. The correct formula is BaF2.
2.72(a)The copper ion has a 1 charge and bromide has a 1 charge. The correct formula is CuBr.
(b)The manganese ion has a 3 charge and oxide has a 2 charge. The correct formula is Mn2O3.
(c)We have the Hg ion and iodide (I). The correct formula is Hg2I2.
(d)Magnesium ion has a 2 charge and phosphate has a 3 charge. The correct formula is Mg3(PO4)2.
2.73Compounds of metals with nonmetals are usually ionic. Nonmetal-nonmetal compounds are usually molecular.
Ionic:LiF, BaCl2, KCl
Molecular:SiCl4, B2H6, C2H4
2.74Compounds of metals with nonmetals are usually ionic. Nonmetal-nonmetal compounds are usually molecular.
Ionic:NaBr, BaF2, CsCl.
Molecular:CH4, CCl4, ICl, NF3
2.75Strategy:When naming ionic compounds, our reference for the names of cations and anions are Tables 2.8 and 2.9 of the text. Keep in mind that if a metal can form cations of different charges, we need to use the Stock system. In the Stock system, Roman numerals are used to specify the charge of the cation. The metals that have only one charge in ionic compounds are the alkali metals (1), the alkaline earth metals (2), Ag, Zn2, Cd2, and Al3.
When naming acids, binary acids are named differently than oxoacids. For binary acids, the name is based on the nonmetal. For oxoacids, the name is based on the polyatomic anion. For more detail, see Section 2.7 of the text.
Solution:
(a)This is an ionic compound in which the metal cation (K) has only one charge. The correct name is potassium dihydrogen phosphate.
(b)This is an ionic compound in which the metal cation (K) has only one charge. The correct name is potassium hydrogen phosphate
(c)This is molecular compound. In the gas phase, the correct name ishydrogen bromide.
(d)The correct name of this compound in water is hydrobromic acid.
(e)This is an ionic compound in which the metal cation (Li+) has only one charge. The correct name is lithium carbonate.
(f)This is an ionic compound in which the metal cation (K) has only one charge. The correct name is potassium dichromate.
(g)This is an ionic compound in which the cation is a polyatomic ion with a charge of +1. The anion is an oxoanion with one less O atom than the corresponding –ate ion (nitrate). The correct name isammonium nitrite.
(h)The oxoanion in this acid is analogous to the chlorate ion. The correct name of this compound is hydrogen iodate (in water, iodic acid)
(i)This is a molecular compound. We use a prefix to denote how many F atoms it contains. The correct name isphosphorus pentafluoride.
(j)This is a molecular compound. We use prefixes to denote the numbers of both types of atom. The correct name is tetraphosphorus hexoxide.
(k)This is an ionic compound in which the metal cation (Cd) has only one charge. The correct name is cadmium iodide.
(l)This is an ionic compound in which the metal cation (Sr) has only one charge. The correct name is strontium sulfate.
(m)This is an ionic compound in which the metal cation (Al) has only one charge. The correct name is aluminum hydroxide.
2.76(a)potassium hypochlorite(h)iron(III) oxide
(b)silver carbonater(i)titanium(IV) chloride
(c)nitrous acid(j)sodium hydride
(d)potassium permanganate(k)lithium nitride
(e)cesium chlorate(l)sodium oxide
(f)potassium ammonium sulfate(m)sodium peroxide
(g)iron(II) oxide
2.77Strategy:When writing formulas of molecular compounds, the prefixes specify the number of each type of atom in the compound.
When writing formulas of ionic compounds, the subscript of the cation is numerically equal to the charge of the anion, and the subscript of the anion is numerically equal to the charge on the cation. If the charges of the
cation and anion are numerically equal, then no subscripts are necessary. Charges of common cations and anions are listed in Tables 2.8 and 2.9 of the text. Keep in mind that Roman numerals specify the charge of the cation, not the number of metal atoms. Remember that a Roman numeral is not needed for some metal cations, because the charge is known. These metals are the alkali metals (1), the alkaline earth metals (2), Ag, Zn2, Cd2, and Al3.
When writing formulas of oxoacids, you must know the names and formulas of polyatomic anions (see Table 2.9 of the text).
Solution:
(a)Rubidium is an alkali metal. It only forms a 1 cation. The polyatomic ion nitrite, NO, has a 1 charge. Because the charges on the cation and anion are numerically equal, the ions combine in a one-to-one ratio. The correct formula is RbNO2.
(b)Potassium is an alkali metal. It only forms a 1 cation. The anion, sulfide, has a charge of 2. Because the charges on the cation and anion are numerically different, the subscript of the cation is numerically equal to the charge on the anion, and the subscript of the anion is numerically equal to the charge on the cation. The correct formula isK2S.
(c)Sodium is an alkali metal. It only forms a +1 cation. The anion is the hydrogen sulfide ion (the sulfide ion plus one hydrogen), HS. Because the charges are numerically the same, the ions combine in a one-to-one ratio.The correct formula is NaHS.
(d)Magnesium is an alkaline earth metal. It only forms a +2 cation. The polyatomic phosphate anion has a charge of 3, PO. Because the charges on the cation and anion are numerically different, the subscript of the cation is numerically equal to the charge on the anion, and the subscript of the anion is numerically equal to the charge on the cation. The correct formula is Mg3(PO4)2. Note that for its subscript to be changed, a polyatomic ion must be enclosed in parentheses.
(e)Calcium is an alkaline earth metal. It only forms a 2 cation. The polyatomic ion hydrogen phosphate, HPO, has a 2 charge. Because the charges are numerically the same, the ions combine in a one-to-one ratio. The correct formula is CaHPO4.
(f)Potassium is an alkali metal. It only forms a 1 cation. The polyatomic ion dihydrogen phosphate, H2PO, has a 1 charge. Because the charges on the cation and anion are numerically equal, the ions combine in a one-to-one ratio. The correct formula is KH2PO4.
(g)This is a molecular compound. The Greek prefixes tell you the number of each type of atom in the molecule: no prefix indicates 1 and hepta- indicates 7. The correct formula is IF7.
(h)The polyatomic ion ammonium, NH, has a 1 charge and the polyatomic ion sulfate, SO, has a 2 charge. To balance the charge, we need 2 NH cations. The correct formula is (NH4)2SO4.
(i)Silver forms only a +1 ion. The perchlorate ion, ClO, has a charge of 1. Because the charges are numerically the same, the ions combine in a one-to-one ratio. The correct formula isAgClO4.
(j)This is a molecular compound. The Greek prefixes tell you the number of each type of atom in the molecule: no prefix indicates 1 and tri- indicates 3. The correct formula is BCl3.
(k)This is a molecular compound. The Greek prefixes tell you the number of each type of atom in the molecule. The correct formula is SeF6.
2.78(a)CuCN (b)Sr(ClO2)2 (c)HBrO4(aq) (d)HI(aq) (e)Na2(NH4)PO4
(f) PbCO3 (g)SnF2 (h)P4S10 (i)HgO (j)Hg2I2 (k)SeF6
2.79(a)Mg(NO3)2(b)Al2O3(c)LiH(d)Na2S
2.80(a)one green sphere, one red sphere(b)one green sphere, two red spheres (c) three green spheres,
two red spheres (d) two green spheres, one red sphere
2.81acid: compound that produces H+; base: compound that produces OH; oxoacids: acids that contain oxygen;
oxoanions: the anions that remain when oxoacids lose H+ ions; hydrates: ionic solids that have water molecules in their formulas.
2.82Uranium is radioactive. It loses mass because it constantly emits alpha () particles.
2.83(c) Changing the electrical charge of an atom usually has a major effect on its chemical properties. The two electrically neutral carbon isotopes should have nearly identical chemical properties.
2.84The number of protons 65 35 30. The element that contains 30 protons is zinc, Zn. There are two fewer electrons than protons, so the charge of the cation is 2. The symbol for this cation is Zn2.
2.85Atomic number 127 74 53. This anion has 53 protons, so it is an iodide ion. Since there is one more
electron than protons, the ion has a 1 charge. The correct symbol is I.
2.86(a)Species with the same number of protons and electrons will be neutral. A, F, G.
(b)Species with more electrons than protons will have a negative charge. B, E.
(c)Species with more protons than electrons will have a positive charge. C, D.
(d)A: B: C: D: E: F: G:
2.87NaCl is an ionic compound; it doesn’t consist of molecules.
2.88Yes. The law of multiple proportions requires that the masses of sulfur combining with phosphorus must be in the ratios of small whole numbers. For the three compounds shown, four phosphorus atoms combine with three, seven, and ten sulfur atoms, respectively. If the atom ratios are in small whole number ratios, then the mass ratios must also be in small whole number ratios.
2.89The species and their identification are as follows:
(a)SO2molecule and compound(g)O3element and molecule
(b)S8element and molecule(h)CH4molecule and compound
(c)Cselement(i)KBrcompound, not molecule
(d)N2O5molecule and compound(j)Selement
(e)Oelement(k)P4element and molecule
(f)O2element and molecule(l)LiFcompound, not molecule
2.90(a)This is an ionic compound. Prefixes are not used. The correct name is barium chloride.
(b)Iron has a 3 charge in this compound. The correct name is iron(III) oxide.
(c)NO2 is the nitrite ion. The correct name is cesium nitrite.
(d)Magnesium is an alkaline earth metal, which always has a 2 charge in ionic compounds. The roman numeral is not necessary. The correct name is magnesium bicarbonate.
2.91All masses are relative, which means that the mass of every object is compared to the mass of a standard
object (such as the piece of metal in Paris called the "standard kilogram"). The mass of the standard object is determined by an international committee, and that mass is an arbitrary number to which everyone in the scientific community agrees.
Atoms are so small it is hard to compare their masses to the standard kilogram. Instead, we compare atomic masses to the mass of one specific atom. In the 19th century the atom was 1H, and for a good part of the 20th century is was 16O. Now it is 12C, which is given the arbitrary mass of 12 amu exactly. All other isotopic masses (and therefore average atomic masses) are measured relative to the assigned mass of 12C.
2.92(a)Ammonium is NH, not NH. The formula should be (NH4)2CO3.
(b)Calcium has a 2 charge and hydroxide has a 1 charge. The formula should be Ca(OH)2.
(c)Sulfide is S2−, not SO. The correct formula is CdS.
(d)Dichromate is Cr2O, not Cr2O. The correct formula is ZnCr2O7.
2.93Symbol
Protons526157986
Neutrons62816117136
Electrons524187986
Net Charge02300
2.94(a)Ionic compounds are typically formed between metallic and nonmetallic elements.
(b)In general the transition metals, the actinides and lanthanides have variable charges.
2.95(a)Li, alkali metals always have a 1 charge in ionic compounds
(b)S2
(c)I, halogens have a 1 charge in ionic compounds
(d)N3
(e)Al3, aluminum always has a 3 charge in ionic compounds
(f)Cs, alkali metals always have a 1 charge in ionic compounds
(g)Mg2, alkaline earth metals always have a 2 charge in ionic compounds.
2.96The symbol 23Na provides more information than 11Na. The mass number plus the chemical symbol identifies a specific isotope of Na (sodium) while combining the atomic number with the chemical symbol tells you nothing new. Can other isotopes of sodium have different atomic numbers?
2.97The binary Group 7A element acids are: HF, hydrofluoric acid; HCl, hydrochloric acid; HBr, hydrobromic acid; HI, hydroiodic acid. Oxoacids containing Group 7A elements (using the specific examples for chlorine) are: HClO4, perchloric acid; HClO3, chloric acid; HClO2, chlorous acid: HClO, hypochlorous acid.
Examples of oxoacids containing other Group A-block elements are: H3BO3, boric acid (Group 3A); H2CO3, carbonic acid (Group 4A); HNO3, nitric acid and H3PO4, phosphoric acid (Group 5A); and H2SO4, sulfuric acid (Group 6A). Hydrosulfuric acid, H2S, is an example of a binary Group 6A acid while HCN, hydrocyanic acid, contains both a Group 4A and 5A element.
2.98(a)C2H2, CH.(b)C6H6, CH.(c)C2H6, CH3.(d)C3H8, C3H8.
2.99(a)Isotope